An image processing apparatus sets a predetermined reading resolution for acquiring image data and a division size for performing a predetermined process on the image data and extract a unique portion from image data resulting from performing the predetermined process. The predetermined process includes an averaging process on the image data for each of division areas obtained by dividing the image data by the predetermined division size, a quantization process for quantizing values obtained by the averaging process, and an addition process for adding values obtained by the quantization process, that are different in the dividing way of the image data in the averaging process. The setting unit sets the predetermined reading resolution so as to make the width of the unique portion in the image correspond to N read pixels (N≥2), and sets the division size so as to have a width larger than N pixels.